The Xanthophyta include more than 600 species. Members of this group are photosynthetic organisms which live primarily in freshwater, though some are found in marine waters, in damp soil, or on tree trunks. They generally are not abundant when they are found at all, and many species have only been found once. Despite this, they are the dominant producers in some salt marshes, and some, like Tribonema, are cosmopolitan in their distribution.
Unlike the other Chromista, Xanthophyta completely lack the brown pigment fucoxanthin. Like these other chromists however, they lack chlorophyll b, and instead have chlorophyll c. This gives them a characteristic yellowish-green color, as opposed to the golden color or their relatives, which may make them difficult to recognize as chromists.
Many xanthophytes produce a cell wall, though it is not composed of cellulose (as in plants) or of chitin (as in fungi). In fact, the cell wall composition of these protists is still completely unknown, though it is known that cysts in this group will often contain silica in their walls. The walls are often, but not always, compsed of two overlapping cylindrical halves which fit together, one slightly inside the other.
Xanthophytes may be sessile or free-living. Most are flagellated unicells, but many are colonial, living as naked cells in a gelatinous envelope. There are also filamentous forms, which produce long chains of cells, and there are coenocytic forms, such as the water-felt Vaucheria. These coenocytes are tubular multinucleated filaments with no internal partitioning into cells. Some orders may live as an amoeboid stage, living on or within plants; in ways similar to another chromist group, the slime nets. However, it is currently thought that the Xanthophyta are more closely related to the brown algae than to any other group.
Sexual reproduction is known only in two genera: Botrydium, in which the sex cells are isogamous, and Vaucheria, in which the cells are oogamous. Most reproduction is asexual, and this is accomplished by a wide variety of means, including fragmentation of filaments, but often involving the production of some kind of spore. Spores may be flagellated and free-swimming (zoospores), or they may be non-flagellated (aplanospores). Spores are formed internally, being released by rupture of the old cell wall.
There are no positively known fossils of the Xanthophyta, though spores and rock deposits of the Miocene and Pleistocene have been attributed to this group. It is likely that the lack of a fossil record results more from the fact that their cyst morphology is poorly studied, rather than from a lack of preservation, since the silicified cysts of this group ought to be found.
Visit Craig Schneider's page on the Vaucheria of Connecticut.
Handbook of Protoctista, ed. by L. Margulis et al., 1990 Jones and Bartlett, chapter 34 by David J. Hibberd.